1. Field of the Invention
The present invention is related to a process for growing a crystal comprising more than three elements (hereinafter, referred as a multi-component crystal), more particularly a process for growing a multi-component crystal having reduced variation of composition in crystal with a higher yield.
2. Description of the Related Art
Single crystals of compound semiconductor, oxide or the like are used in a wide variety of solid-state devices in the field of microelectronics, optoelectronics etc.
Recently, new materials consisting of a plurality of elements have been developed in order to improve functions of the devices and to give them additional functions. For example, mixed crystals or solid solutions consisting of more than three elements are used, since physical properties (for example, the lattice constant, the band gap, the light reflectance, the thermal expansion coefficient, mobility of electrons and chemical properties (surface energy, the resistance to chemicals etc.) of two-component compound semiconductors are fixed unconditionally according to elements of which the two-component compound semiconductor is made, and hence these properties can not be improved or controlled artificially.
The merits of such multi-component mixed crystal are that there is a wide range of selection in their constituent elements, their proportions or the like and hence it is possible to obtain a variety of compound semiconductors having desired properties. This fact is the same in case of a single crystal of oxide which consists of more than three constituent elements and several kinds of which have been proposed.
The multi-component crystal can be prepared by a variety of methods. In a method in which multi-component crystal is grown from the melt of material composed of plural elements in a sealed reaction tube, Vertical Bridgman method, Traveling Heater method or the like can be utilized.
FIGS. 7 and 8 illustrates an examples of the process for growing crystal of multi-component compound semiconductor by the conventional Vertical Bridgman Method, in which FIG. 7 shows a stage of mixed crystal growth and FIG. 8 shows the temperature distribution of a furnace employed for this process. Now, we will explain an outline of this process by describing a case in which a mixed crystal having a composition: A.sub.1-x B.sub.x C (wherein, A, B, C denote consistent elements, and 0&lt;x&lt;1) is produced.
After a mixed crystal (a seed crystal) having the composition of A.sub.1-x B.sub.x C is placed at the bottom of a reaction tube 51 made of quartz etc., predetermined amounts of the material 52 for a miaxed crystal, for example, A.sub.1-x C.sub.1-x and B.sub.x C.sub.x are charged onto the seed crystal. Then, the pressure in the reaction tube 51 is reduced to create a vacuum condition. The resulting reaction tube 51 under vacuum is placed at the determined position in a furnace (not shown) having the temperature distribution presented in FIG. 8 to melt the materials 52 of mixed crystal. The seed crystal starts to grow with being in contact with the melt 52 of the materials. Thereafter, the reaction tube 51 is moved slowly downward in the direction of an arrow shown in FIG. 7, so that the liquid-solid interface moves upwardly in reaction tube 51. In this manner, a mixed crystal is prepared by growing the resulting crystal part 53.
FIGS. 9 and 10 illustrate examples of the process for growing a mixed crystal for compound semiconductors by the conventional Traveling Heater Method, in which FIG. 9 shows briefly a stage of mixed crystal growth and FIG. 10 shows the temperature distribution of a furnace employed for this process. In this example, the starting materials, for instance, A.sub.1-x C.sub.1-x (64) and B.sub.x C.sub.x (65) are placed in the manner to confront each other along the vertical axis of reaction tube 61 and the temperature distribution is controlled to partially form the molten zone 62. Other operations of this process for producing a mixed crystal 63 are substantially the same as in the Vertical Bridgman Method.
In the prior art, however, it is difficult to produce a uniform crystal having no lattice defects such as vacancies and also it is impossible to control a stoichiometrical composition, because a part of the consistent element in the starting material having a higher vapour pressure evaporates when the starting material is melted. In fact, the starting material of the mixed crystal and also the resulting mixed crystal itself are mostly such compounds having higher vapour pressure under a molten condition.
For example, when a mixed crystal of Cd.sub.1-x Zn.sub.x Te is produced according to the prior art, its components Cd and Zn having higher vapour pressures are apt to evaporate and hence the resulting crystal contains vacancies of Cd and Zn and becomes mostly a P-type conduction type. Thus, the prior art is thoroughly imperfect in such aspect as control of the composition in a crystal and stoichiometrical control. Therefore, in the conventional technique, it is difficult to produce multi-component crystal having stable or constant properties with a higher yield due to fluctuation of composition and deviation of stoichiometry.
Accordingly, it is an object of the present invention to provide a method for growing a multi-component type crystal which is stoichiometrically stable or constant in composition with a higher yield and without changing conventional manufacturing facilities or units.